Thiopurine methyltransferase genotype and activity cannot predict outcomes of azathioprine
maintenance therapy for antineutrophil cytoplasmic antibody associated vasculitis
Hessels, Arno C; Rutgers, Abraham; Sanders, Jan Stephan F; Stegeman, Coen A
Published in:PLoS ONE DOI:
10.1371/journal.pone.0195524
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Hessels, A. C., Rutgers, A., Sanders, J. S. F., & Stegeman, C. A. (2018). Thiopurine methyltransferase genotype and activity cannot predict outcomes of azathioprine maintenance therapy for antineutrophil cytoplasmic antibody associated vasculitis: A retrospective cohort study. PLoS ONE, 13(4), [e0195524]. https://doi.org/10.1371/journal.pone.0195524
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Thiopurine methyltransferase genotype and
activity cannot predict outcomes of
azathioprine maintenance therapy for
antineutrophil cytoplasmic antibody
associated vasculitis: A retrospective cohort
study
Arno C. Hessels1*, Abraham Rutgers2, Jan Stephan F. Sanders1, Coen A. Stegeman1
1 Department of Internal Medicine, Division of Nephrology, University of Groningen, University Medical
Center Groningen, Groningen, The Netherlands, 2 Department of Rheumatology and Clinical Immunology, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands
*a.c.hessels@umcg.nl
Abstract
Objective
Azathioprine is a widely used immunosuppressive drug. Genetic polymorphisms and activity of the enzyme thiopurine methyltransferase (TPMT) have been associated with azathioprine efficacy and toxicity in several populations. We investigated whether these associations also exist for ANCA associated vasculitis (AAV) patients, who receive azathioprine mainte-nance therapy after remission induction with cyclophosphamide.
Methods
207 AAV patients treated with cyclophosphamide induction and azathioprine maintenance therapy were included and followed for 60 months. TPMT genotype and tertiles of TPMT activity were compared to relapse free survival and occurrence of adverse events, particu-larly leukopenia. Multivariable regression was performed to account for confounders.
Results
In univariable analysis, relapse free survival was not significantly associated with TPMT genotype (P = 0.41) or TPMT activity (P = 0.07), although it tended to be longer in lower ter-tiles of TPMT activity. There was no significant association of TPMT genotype and activity with occurrence of any adverse event. In multiple regression, leukocyte counts at the end of cyclophosphamide induction were related to risk of leukopenia during azathioprine therapy [P<0.001; OR 0.54 (95% CI 0.43–0.68)] and risk of relapse during follow-up [P = 0.001; HR 1.17 (95% CI 1.07–1.29)] irrespective of TMPT genotype or activity.
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Citation: Hessels AC, Rutgers A, Sanders JSF, Stegeman CA (2018) Thiopurine methyltransferase genotype and activity cannot predict outcomes of azathioprine maintenance therapy for antineutrophil cytoplasmic antibody associated vasculitis: A retrospective cohort study. PLoS ONE 13(4): e0195524.https://doi.org/10.1371/journal. pone.0195524
Editor: Marieke J. H. Coenen, Radboud university medical center, NETHERLANDS
Received: September 12, 2017 Accepted: March 23, 2018 Published: April 9, 2018
Copyright:© 2018 Hessels et al. This is an open access article distributed under the terms of the
Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability Statement: Data cannot be made publicly available due to patient privacy concerns and the restrictions for data use agreement signed by the authors of this study. The authors note that the Medical Ethical Committee of the University Medical Center Groningen (METc UMCG) does not hold data for researchers. A formal Data Access Committee also does not yet exist for this organization. Due to these limitations, the authors have arranged the following for data access
Conclusion
TPMT genotype and activity were not independent predictors of relapse, and could not predict leukopenia or other adverse effects from azathioprine. Leukocyte counts after cyclophosphamide induction were related to both outcomes, implying a greater influ-ence of cyclophosphamide response compared to azathioprine and TPMT in AAV patients.
Introduction
Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) refers to a group of primary small-vessel vasculitides. The most common forms are granulomatosis with poly-angiitis (GPA, formerly Wegener’s Granulomatosis) and microscopic polypoly-angiitis (MPA).[1] Induction treatment with cyclophosphamide, rituximab or mycophenolate mofetil combined with corticosteroids can achieve remission in most AAV patients and reduce mortality, but is associated with considerable toxicity.[2–4] For this reason, patients switch to less toxic mainte-nance therapy after achieving remission, most frequently azathioprine.[3–5] Even with azathi-oprine maintenance therapy, there is a risk of potentially severe adverse effects, most
frequently leukopenia and infection.[5,6] In recent years, there has been increasing interest in personalised medicine whereby treatment is adjusted based upon characteristics of an individ-ual patient, thereby optimizing efficacy and reducing toxicity.
Azathioprine and its metabolite 6-mercaptopurine are converted via several enzymatic steps into 6-thioguanine nucleotides (6-TGN), the active metabolites responsible for the immunosuppresive effect and myelotoxicity. The enzyme thiopurine methyltransferase (TPMT) methylates several metabolites along the enzymatic pathway, thereby reducing the amount of 6-TGN formed.[7,8] Several polymorphisms of the gene encoding TPMT have been identified, each resulting in decreased activity of the enzyme.[7–9] Approximately 89% of Caucasians are homozygous for wildtypeTPMT alleles corresponding with normal or high
activity.11% carry a wildtype and a variant allele corresponding with intermediate TPMT activity. Very few individuals (0.3%) are homozygous or compound heterozygous for variant alleles resulting in absence of TPMT activity.[8,10]
Studies in several populations, mainly inflammatory bowel disease (IBD), have shown that
TPMT variant alleles and lower TPMT activity are associated with a higher risk of bone
mar-row toxicity.[7,11–13] Patients carrying two variantTPMT alleles are especially at risk for
severe myelotoxicity[11] and require either a 10-fold lower dose or alternative therapy (e.g. methotrexate).[6,8,14] For patients with intermediate TPMT activity carrying one variant allele, more controversy exists. Several meta-analyses have shown an increased risk of myelo-toxicity in these patients.[12,15] While clinical trials did not find a significant reduction of tox-icity when adjusting azathioprine or 6-mercaptopurine dose on TPMT genotype or activity, [14,16,17] post-hoc analysis showed a significant reduction of myelotoxicity within carriers of a variant allele.[14]
The aforementioned studies mainly involve patients with IBD, who receive azathioprine as their main treatment drug. Since AAV patients receive azathioprine after an induction phase with cyclophosphamide,[4] the influence of TPMT might be smaller and less relevant in this population.
The aim of this study was to see whether TPMT genotype and activity are associated with bone marrow toxicity and risk of relapse in AAV patients treated with azathioprine maintenance requests: Dr. Elisabeth Brouwer (a researcher who
did not collaborate in the study and is not an author on the article) will hold the data and will serve as the point of contact for data access requests. Interested, qualified researchers can contact Dr. Brouwer ate.brouwer@umcg.nl. On request, Dr. Brouwer will contact the "Loket Contract Research", a legal department within the UMCG concerned with research contracts. They will review whether the data request can be honoured based on the nature of the request, the Informed Consent of the study and current Privacy Laws in the Netherlands. If the request is accepted, Dr. Brouwer will send a Data Transfer Agreement to the requestor ensuring responsible use of the data. After that, data will be transferred to the requestor for the required and agreed upon period of time.
Funding: The authors received no specific funding for this work.
Competing interests: The authors have declared that no competing interests exist.
therapy. We expanded on an earlier study in our population[18] by taking into account the influ-ence of cyclophosphamide induction therapy on these outcomes.
Patients and methods
Patients
For this retrospective cohort study, 377 patients, diagnosed with GPA, MPA or Renal Limited Vasculitis (RLV) between September 1984 and August 2013 in the University Medical Center Groningen (UMCG) and treated with oral cyclophosphamide following diagnosis, were con-sidered for inclusion. Patients were included if they switched to azathioprine after induction of remission and had a follow-up of at least a year. All patients have given written informed con-sent according to the Declaration of Helsinki for participation in a large cohort study investi-gating biomarkers (including TPMT) in relation to disease outcome in AAV. Ethical approval for the study was granted by the local Medical Ethical Committee of the University Medical Center Groningen (NL29354.042.10).
Treatment protocol
Following diagnosis, all patients were treated with oral cyclophosphamide (1.5–2.0 mg/kg/ day) combined with prednisolone (1mg/kg/day, max 60mg/day). Prednisolone dose was reduced according to a standard schedule (S1 Table). After 3 months of stable remission, all patients switched to maintenance therapy with azathioprine. The starting dose was a conver-sion from cyclophosphamide dose to the same azathioprine dose. The target azathioprine dose was 1.5–2.0mg/kg/day. Starting 12 months after diagnosis, azathioprine dose was reduced by 25 mg/day every 3 months. Leukocyte counts were measured 1 week after starting azathioprine and at least every 4 weeks thereafter. During treatment,cyclophosphamide and azathioprine dose were adjusted based on leukocyte counts (goal: leukocytes 4.0109
/l) in accordance with the CYCAZAREM protocol,[5] and occurrence of infections.
Data collection
All information was collected from the patients’ records. For all patients, demographic, disease and treatment characteristics, as well as clinical outcome data were registered. Diagnosis was based on the 2012 Chapel Hill Consensus Conference definitions.[1] Disease activity at diag-nosis was scored using the Birmingham Vasculitis Activity Score 1 (BVAS-1).[19] Patients were screened for the presence of ANCA using indirect immune fluorescence (IIF), and ANCA-specificity was determined using ELISA.
The primary endpoints of the study were relapse-free survival in months and leukopenia. Relapse was defined as new or worsening disease activity requiring dose increase or switch of immunosuppressive medication. Leukopenia was defined as leukocyte count <4.010^9/l.[20]
Secondary categorical endpoints were moderate leukopenia (leukocyte count <3.010^9/l),
[20] macrocytic anemia (Hb <7.5 for females and <8.0 for males; MCV>96fl), hepatotoxicity (ASAT and/or ALAT >2x upper limit of normal, or AF >125 U/l), infection (requiring hospi-talisation and/or antibiotics, or opportunistice.g. CMV, VZV, HSV, and/or pneumocystis
jiro-vecii pneumonia). These endpoints were scored if they occurred at any time during azathioprine therapy. Secondary continuous endpoints were leukocyte counts 3,6,9 and 12 months after switch to azathioprine, and the [leukocyte(109
/l)][azathioprine(mg/kg/d)]
product 3,6,9 and 12 months after switch as a measure of sensitivity for azathioprine-induced bone marrow depression.[18] Diagnosis, ANCA specificity, age at diagnosis, baseline serum creatinine, co-trimoxazole use at switch to azathioprine (none, prophylactic or therapeutic
dose), leukocyte count at switch and duration of azathioprine therapy were registered for their potential influence on relapse. Factors registered for their potential influence on risk of leuko-penia include prednisolone dose at switch, cyclophosphamide dose at switch, leukocyte count at switch and azathioprine dose at switch. Prednisolone dose during azathioprine therapy was registered to account for its influence on leukocyte counts.
Measurement of
TPMT genotype and TPMT activity
Four variants of theTPMT gene, located on chromosome 6, were determined using PCR, as
described by Yates et al.[9] The genetic variants wereTPMT2 (G!C translocation at nucleotide
238),TPMT3A (460G!A and 719A!G),TPMT3B (460G!A), andTPMT3C (719A!G).
TPMT activity was determined by adding 6-thioguanine to human erythrocytes in vitro, and measuring the amount of 6-methylthioguanine formed (TPMT catalyses this reaction), expressed in nmol 6-methylthioguanine formed per gram haemoglobin per hour (nmol/gHb/ hr).[21] In the majority of patients (67%), TPMT genotype and activity were measured after starting azathioprine treatment. In some patients (33%), these were measured before starting azathioprine. The date of blood withdrawal for TPMT measurement was registered for all patients.
Statistics
Statistical analysis was done using SPSS Statistics 22 (IBM Corporation, New York, US). Data are shown as median + interquartile range (IQR) or number + percentage. A two-sided P<0.05 was considered statistically significant. Univariate analysis was performed for
TPMT genotypes and tertiles of TPMT activity (tertiles determined based on equal numbers of
patients per group) using a Log Rank test for relapse free survival (up to 60 months after diag-nosis), Fisher’s exact test or Chi Square test for risk of adverse events, and Mann-Whitney or Kruskal-Wallis test for leukocyte count and [leukocyte][azathioprine] product. Multivariate
analysis was performed with relapse-free survival, risk of leukopenia and leukocyte counts as outcome variable, using Cox regression, logistic regression and linear regression, respectively. Possible predictors in the analysis wereTPMT genotypes, tertiles of TPMT activity and
poten-tial influencing factors for the respective outcomes mentioned under ‘data collection’. A for-ward stepwise model was used, where variables were included as covariates based on a univariate P<0.05 and excluded on a multivariate P-value >0.10. Non-proportional hazards for predictors in Cox regression were accounted for by adding a time-by-predictor interaction variable to the model.[22]
Results
Patients and TPMT genotype and activity
207 patients were included in the analysis.(Fig 1) Demographic and disease characteristics, as well as distribution ofTPMT genotypes and TPMT activity, are shown inTable 1.
TPMT activity approximated a Gaussian distribution (Fig 2). TPMT activity was signifi-cantly lower in carriers ofTPMT3A (43.9; IQR 40.6–49.5 nmol/gHb/hr) andTPMT3C (43.5 nmol/gHb/hr) compared to patients with a homozygous normal genotype (81.4; IQR 73.5– 92.2) nmol/gHb/hr)(P<0.001). TPMT activity was divided in tertiles based on the number of patients. The lowest tertile (T1) contains patients with TPMT activity 74.5, the second tertile (T2) contains patients with TPMT activity 74.6–86.4 and the highest tertile (T3) contains patients with TPMT activity 86.5 nmol/gHb/hr. None of the characteristics differed between
significantly lower in patients with heterozygousTPMT genotype (1.0, IQR 0.7–1.4 mg/kg/
day) compared to patients with normal genotype (1.5, IQR 1.1–1.8 mg/kg/day) (P = 0.001). Azathioprine starting dose was not significantly related to measurement of TPMT status prior to (n = 68) or after (n = 139) start of azathioprine therapy (P = 0.92), even when specifically analyzing patients with a heterozygousTPMT genotype (P = 0.28). As expected from the
treat-ment protocol, azathioprine starting dose showed a strong positive correlation with cyclophos-phamide dose at switch (Rho = 0.70, P<0.001). No patients were treated with
6-mercaptopurin.
Relapse free survival
Within 5 years after diagnosis, 6 of 19 patients (32%) with heterozygousTPMT genotype
expe-rienced a relapse, compared to 84 of 188 patients (45%) with normalTPMT genotype. There
was no significant difference in relapse-free survival (P = 0.30) betweenTPMT genotypes (Fig 3A).
Tertiles of TPMT activity showed a negative trend with relapse-free survival (P = 0.07) (Fig 3B). In the lowest tertile (T1), 34% of patients experienced relapse within 5 years, compared to 47% in the middle tertile (T2) and 50% in the highest tertile (T3). Relapse free survival was still not significantly related toTPMT genotype (P = 0.39) and tertiles of TPMT activity (P = 0.21)
after exclusion of patients intolerant to azathioprine.
In Cox regression,TPMT genotypes (P = 0.39), tertiles of TPMT activity (P = 0.24),
co-tri-moxazole use (P = 0.15), age (P = 0.69) and diagnosis (P = 0.94) were not significantly related to the occurrence of relapse. ANCA specificity (P = 0.003), duration of azathioprine therapy (P<0.001), serum creatinine at baseline (P = 0.007) and leukocyte count at switch (P = 0.001) were significantly associated with relapse. Risk of relapse was higher for PR3-ANCA positive patients (Hazard Ratio (HR)3.1; 95% CI 1.5–6.6), and for patients with a higher leukocyte count after cyclophosphamide induction (HR 1.17; 95% CI 1.07–1.29). Risk of relapse was Fig 1. Flow-chart of patient selection.
lower for patients with a longer duration of azathioprine maintenance (HR 0.91; 95%CI 0.87– 0.96), and patients with a baseline creatinine >1.24 mg/dl (HR 0.5, 95%CI 0.3–0.8). The inter-action [azathioprine duration][time] was significant (P = 0.008) and indicated a declining
protective effect of azathioprine duration over time (HR 1.002, 95%CI 1.000–1.003). The same variables remained significant after exclusion of patients intolerant to azathioprine (S2 Table).
Adverse events
In total, 35 patients (16%) were intolerant to azathioprine. 17 patients (8%) had gastro-intesti-nal complaints, 17 (8%) had a febrile hypersensitivity reaction, and 1 patient (1%) had a rash. Intolerance to azathioprine was not related toTPMT genotype (P = 0.11) or tertiles of TPMT
activity (P = 0.39). There was no significant difference betweenTPMT genotypes in occurrence
of mild or moderate leukopenia (Table 2). Occurrence of mild or moderate leukopenia also did not significantly differ between tertiles of TPMT activity (Table 3).
The lowest measured leukocyte count was 1.410^9
/l. Two patients had concomitant infec-tions (PCP pneumonia and CMV antigenemia, candida stomatitis and PCP pneumonia, respectively). In 12 patients with moderate leukopenia, azathioprine dose was reduced and in 9 patients azathioprine was (temporarily) discontinued. In all except 3 patients, moderate leu-kopenia was incidental. In the others durations were 5, 7 and 35 days before leukocyte counts were >3.0109
/l.
Table 1. Patient characteristics.
Characteristics N (%)/mean (SD)/median (IQR)
Female 94 (45%)
Age at diagnosis (years) 57 (46–66)
Diagnosis GPA 152 (73%)
PR3-ANCA 150 (73%)
BVAS at diagnosis 18 (13–24)
Serum creatinine at baseline (mg/dl) (n = 182/207) 1.24 (0.89–2.84) Leukocyte count at switch (109
/l) (n = 187/207) 6.6 (5.5–8.3) Co-trimoxaxole use at switch (n = 194/207)
• None 41 (21%)
• Prophylactic dose (480 mg/day) 130 (67%)
• Therapeutic dose (1920 mg/day) 23 (12%)
Cyc start dose (mg/kg/day) (n = 191/207) 1.7 (0.4) Duration of cyc therapy (months) (n = 206/207) 5 (4–6) Prednisolone switch dose (mg/day)(n = 188/207) 12.5 (8.1–20.0) Azathioprine switch dose (mg/kg/day) (n = 195/207) 1.4 (0.5) Duration of azathioprine therapy (months) (n = 204/207) 17 (7–24)
Follow-up time (months) 54 (32–60)
TPMT genotype
• No variant (1/1) 188 (91%)
•TPMT1/3A 16 (8%)
•TPMT1/3C 3 (1%)
TPMT activity (nmol/gHb/hr) 80.0 (17.9)
GPA granulomatosis with polyangiitis; MPA microscopic polyangiitis; RLV renal limited vasculitis; PR3 proteinase 3; MPO myeloperoxidase; BVAS Birmingham Vasculitic Activity Score; Cyc cyclophosphamide; TPMT thiopurine methyltransferase
Azathioprine starting dose was not significantly different between patients with (median 1.6; IQR 1.2–1.8 mg/kg) and without (median 1.4; IQR 0.9–1.8 mg/kg) mild leukopenia (P = 0.08), neither between patients with (median 1.5; IQR 1.1–1.7 mg/kg) or without (median 1.5; IQR 1.1–1.8) moderate leukopenia (P = 0.65). The same goes for patients with and without macrocytic anemia (P = 0.09), liver toxicity (P = 0.26) and infections (P = 0.69).
In logistic regression, TPMT genotype and activity were not significantly related to leuko-penia during azathioprine therapy. Prednisolone dose and cyclophosphamide dose at switch were also not significant. Leukocyte count at switch (i.e. at the end of induction therapy with
cyclophosphamide) remained in the model as a significant predictor of leukopenia (P<0.001), as well as azathioprine dose at switch (P = 0.04) with a higher risk of leukopenia during azathi-oprine therapy in patients with a lower leukocyte count at the end of cyclophosphamide ther-apy (Odds Ratio (OR) 0.54; 95% CI 0.43–0.68), and for patients with a higher starting dose of azathioprine (OR 2.2; 95% CI 1.0–4.6). See alsoS3 Table.
Leukocyte counts 3, 6, 9 and 12 months after switch to azathioprine were not significantly different betweenTPMT genotypes or tertiles of TPMT activity. The [leukocyte]
[azathio-prine] product was significantly lower for heterozygous patients 3 months after switch
(P = 0.03) but not at later time points (Fig 4A). The [leukocyte][azathioprine] product did not
significantly differ between tertiles of TPMT activity (Fig 4B).
In multivariate linear regression, after correction for prednisolone dose,TPMT genotype
was still not a significant predictor of leukocyte count or [leukocyte][azathioprine] product at any time point. Tertiles of TPMT activity, on the other hand, were positively related to leuko-cyte counts 3 months (P<0.001; b (regression coefficient of predictor) = 0.74; 95% CI 0.35– 1.14) and 9 months after switch (P = 0.04; b = 0.36; 95% CI 0.02–0.70) after correction for prednisolone dose. Multivariate linear regression also showed a significant association of TPMT activity tertiles with the [leukocyte][azathioprine] product 3 (P = 0.003; b = 1.20; 95%
CI 0.41–2.00), 9 (P = 0.01; b = 0.94; 95% CI 0.20–1.69) and 12 months after switch (P = 0.02; b = 0.76; 95% CI 0.12–1.41). This indicates higher [leukocyte][azathioprine] product and
Fig 2. Distribution of TPMT activity. TPMT activity in nmol/gHb/hr for patients with normal (gray) and heterozygous (black) TPMT genotype. https://doi.org/10.1371/journal.pone.0195524.g002
Fig 3. Relapse free survival forTPMT genotypes and tertiles of TPMT activity. Relapse free survival after start of cyclophosphamide induction therapy. Upper graph (3A): Variant = heterozygousTPMT variant carrier, Normal = normal genotype (wildtype TPMT). Lower graph (3B): T1 = lowest tertile,
T2 = middle tertile, T3 = highest tertile of TPMT activity.
therefore lower sensitivity for azathioprine-induced leukopenia in patients with higher TPMT activity. Prednisolone dose showed a significant positive association with leukocyte counts and the [leukocyte][azathioprine] product at every time point (P0.001).
TPMT genotype was not related to occurrence of macrocytic anemia, liver toxicity,
infec-tion, or intolerance to azathioprine (Table 2). Occurrence of these adverse events was also not significantly different between tertiles of TPMT activity (Table 3).
Discussion
In this study, we found no significant association ofTPMT genotype and TPMT activity with
relapse free survival. TPMT genotype and activity were not related to occurrence of Table 2. Adverse events in relation toTPMT genotype.
Adverse events All n(%) Variant n(%) Normal n(%) P
All azathioprine tolerant patients 172 15 157
Missing data on leukopenia 8 0 8
0.79 Leukopenia 75 (46) 6 (40) 69 (46) • Mild leukopenia (<4109 /l) 54 (33) 4 (27) 50 (34) >0.99 • Moderate leukopenia (<3109/l) 21 (13) 2 (13) 19 (13)
Missing data on macrocytic anemia 10 0 10
0.28
Macrocytic anemia 74 (46) 9 (60) 65 (44)
Missing data on hepatotoxicity 8 0 8
0.26
Hepatotoxicity 26 (16) 4 (27) 22 (15)
Missing data on infections 11 1 10
0.40
Infection 62 (39) 7 (50) 55 (37)
Number of patients experiencing adverse events. All analyses (except for intolerance) have been done only in patients who were not intolerant to azathioprine (n = 172). All = all patients. Variant = heterozygousTPMT variant carrier, Normal = normal TPMT genotype.
Compared betweenTPMT genotypes. https://doi.org/10.1371/journal.pone.0195524.t002
Table 3. Adverse events in relation to TPMT activity.
Adverse events All n(%) T1 n (%) T2 n (%) T3 n (%) P
All azathioprine tolerant patients 172 57 55 60
Missing data on leukopenia 8 1 1 6
0.82
Leukopenia 75 (46) 27(48) 23 (43) 25 (46)
• Mild leukopenia (<4109/l) 54 (33) 17 (31) 20 (37) 17 (31)
0.13 • Moderate leukopenia (<3109/l) 21 (13) 10 (18) 3 (6) 8 (15)
Missing data on macrocytic anemia 10 2 2 6
0.11
Macrocytic anemia 74 (46) 29 (53) 18 (34) 27 (50)
Missing data on hepatotoxicity 8 1 1 6
0.48
Hepatotoxicity 26 (16) 11 (20) 6 (11) 9 (17)
Missing data on infections 11 3 1 7
0.90
Infection 62 (39) 22 (41) 21 (39) 19 (36)
Number of patients experiencing adverse events. All analyses (except for intolerance) have been done only in patients who were not intolerant to azathioprine (n = 172). All = all patients. T1 = first tertile, T2 = second tertile, T3 = third tertile of TPMT activity.
Compared between tertiles of TPMT activity. https://doi.org/10.1371/journal.pone.0195524.t003
Fig 4. [leuko][aza] product over time forTPMT genotypes and tertiles of TPMT activity. [leukocyte][azathioprine] product 3,6,9 and 12 months
after switch to azathioprine.P<0.05. Upper graph (4A): Variant = heterozygousTPMT variant carrier; Normal = normal genotype (wildtype TPMT).
Lower graph (4B): T1 = lowest tertile, T2 = middle tertile, T3 = highest tertile of TPMT activity.
azathioprine related adverse events. Leukocyte counts at the end of cyclophosphamide induc-tion therapy were significantly associated with both relapse free survival and occurrence of leu-kopenia during azathioprine maintenance therapy.
Because azathioprine therapy is associated with a risk of potentially severe adverse events such as bone marrow toxicity, several studies have focused on TPMT genotypes and activity as predictors of these adverse events.[8,15] It has been established mainly in IBD patient popula-tions that patients with a heterozygousTPMT genotype and lower TPMT activity are at
increased risk of developing adverse events,[8] and that pretesting for TPMT has a beneficial effect specifically in the group of patients with one or severalTPMT variants.[14] Although we found that a lower TPMT activity was associated with a lower [leukocyte][azathioprine] prod-uct, indicating an increased sensitivity to azathioprine-induced leukopenia, we did not find an association ofTPMT genotype and activity with bone marrow toxicity in our population of
AAV patients. This might be explained by the fact that AAV patients do not receive azathio-prine as the main treatment drug, but as maintenance therapy after induction therapy with cyclophosphamide.[4] The effect of cyclophosphamide on bone marrow toxicity during azathi-oprine therapy may be greater than the effect of TPMT, as evidenced by the strong association of leukocyte counts after cyclophosphamide treatment with both relapse free survival and leu-kopenia during azathioprine therapy in this study, and the lack of a significant difference in azathioprine starting dose between patients with and without leukopenia. Another explanation may be that leukocyte counts after cyclophosphamide therapy reflect an overall bone marrow susceptibility to the effects of both drugs.
The frequency of leukopenia in or population was relatively high compared to previous reports, such as in the CYCAZAREM trial (30%).[5] The first reason for this is that patients with leukopenia at the start of azathioprine therapy were also scored as having leukopenia dur-ing azathioprine therapy. The second reason is that any leukopenia durdur-ing the full duration of azathioprine therapy was scored, compared to only the first 15 months after switch in the CYCAZAREM trial.[5] When counting only patients that developed leukopenia within 15 months after start of azathioprine, the frequency of leukopenia (<4.0109/l) in our population was 31%, similar to the frequency previously reported.[5]
Theoretically, patients with lower TPMT activity can achieve a higher efficacy of azathio-prine.[8,23] Some studies indeed found an association of TPMT activity with clinical response. [24,25] In a recently published RCT, adjusting azathioprine dose based onTPMT genotype did
not result in a difference in treatment response between intervention and control groups.[14] Although we found that patients withTPMT variant alleles and patients with lower TPMT
activity had a higher relapse free survival, these differences were not significant, especially when taking other predictors of relapse into account. Interestingly, higher leukocyte counts after cyclophosphamide therapy were a strong predictor of relapse. This indicates that response to cyclophosphamide may be a stronger predictor of clinical efficacy than TPMT.
This study has several limitations. First, although 207 patients is an impressive number for a single center study on a rare disease such as AAV, the sample size may be insufficient to detect relevant associations with sufficient power. This is especially true for the analyses on
TPMT genotype, since there are only 19 patients with a heterozygous TPMT genotype in our
study. Second, treating physicians were not blinded to a patient’s TPMT genotype and activity. On the other hand, adjustment of azathioprine dose based on TPMT status was not included in the treatment protocol, and the initial azathioprine dose of patients with heterozygous
TPMT genotype did not differ between patients whether their TPMT genotype and activity
were measured before or after azathioprine therapy. Third, the study was conducted in a ter-tiary referral center, with some patients receiving part of their follow-up elsewhere. This resulted in missing values on adverse effects of 11 patients. The baseline characteristics and
induction treatment of these patients did not significantly differ from those of patients with follow-up during azathioprine therapy. Lastly, the ethnicity for patients was not registered, although over 95% of patients in our study population are estimated to be Caucasian. As we only genotyped for TPMT variants common in Caucasians, some non-Caucasian patients might have reduced TPMT activity resulting from an untypedTPMT variant. This could
theo-retically result in underestimation of the effects fromTPMT genotype.
The study also has several strengths. It was performed in a single center and all patients were treated according to the same protocol, thereby eliminating between-center differences in treatment and followup measurements. Also, compared to an earlier study on TPMT geno-type and activity in AAV patients from our population,[18] this study has a larger sample size (207 compared to 108), has a longer duration of follow-up and includes multivariable analyses to account for induction treatment and other factors influencing disease free survival and risk of adverse events.
In conclusion, TPMT genotype and activity were not related to azathioprine efficacy and toxicity in our retrospective cohort of AAV patients receiving azathioprine maintenance ther-apy. Response to cyclophosphamide, on the other hand, may have a stronger predictive value on these outcomes. This should be confirmed in a sufficiently large multicenter study.
Supporting information
S1 Table. Tapering scheme for prednisolone. † Start tapering earlier when in full remission
for 2weeks, <6wks of therapy. (DOCX)
S2 Table. Cox regression for 5 year relapse free survival. Cox regression analysis for 5 year/
60 month relapse free survival for non-azathioprine intolerant patients (n = 172). Variables for the final model were selected using a forward stepwise method (inclusion if univariate P<0.05, exclusion if multivariate P>0.1). ANCA specificity, duration of azathioprine therapy, creati-nine at baseline and leukocyte count after cyclophosphamide induction therapy were signifi-cantly associated with risk of relapse.P<0.05;P<0.01;P<0.001.
(DOCX)
S3 Table. Logistic regression for risk of leukopenia. Logistic regression for risk of leukopenia
(leukocyte count <4.0109/l) for non-intolerant patients (n = 172). Variables for the final
model were selected using a forward stepwise method (inclusion if univariate P<0.05, exclu-sion if multivariate P>0.1). A higher leukocyte count after cyclophosphamide induction ther-apy was associated with a lower risk of leukopenia, and a higher starting dose of azathioprine was associated with a higher risk of leukopenia during azathioprine therapy.P<0.05;
P<0.01;P<0.001. (DOCX)
Author Contributions
Conceptualization: Abraham Rutgers, Jan Stephan F. Sanders, Coen A. Stegeman. Formal analysis: Arno C. Hessels.
Investigation: Arno C. Hessels, Jan Stephan F. Sanders.
Methodology: Arno C. Hessels, Abraham Rutgers, Coen A. Stegeman.
Project administration: Arno C. Hessels, Abraham Rutgers, Jan Stephan F. Sanders, Coen A.
Supervision: Abraham Rutgers, Jan Stephan F. Sanders, Coen A. Stegeman. Validation: Abraham Rutgers.
Visualization: Arno C. Hessels.
Writing – original draft: Arno C. Hessels.
Writing – review & editing: Abraham Rutgers, Jan Stephan F. Sanders, Coen A. Stegeman.
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